The present invention relates to a method of decreasing an amount of nitrogen oxides from flue gases of a boiler, which nitrogen oxides are generated in the combustion of fuels and air or other oxygen-containing gas. The invention also relates to a steam-generating boiler.
Flue gases of steam-generating boilers, such as a recovery boiler of a chemical pulp mill, are led from the furnace into contact with various heat exchangers, superheaters, boiler bank and water preheaters of the boiler, whereby the heat contained in the gases is recovered in the water, steam or mixture thereof flowing in the heat exchangers. The boiler bank refers to a heat exchanger comprising heat exchange elements, inside which the boiler water to be heated flows. The economizer (preheater) of the boiler refers to a heat exchanger comprising heat exchange elements, inside which the boiler feed water to be heated flows. Free space for flue gas flow remains in the boiler bank and the economizer between the heat exchanger elements. As the flue gas passes by the heat exchanger elements, heat is transferred into the feed water or boiler water flowing inside the elements. From the economizer the flue gases of the boiler are led in a way known per se via a flue gas discharge conduit to gas purification following the boiler, such as an electrostatic precipitator.
FIG. 1 illustrates the construction of a chemical recovery boiler having a furnace 1 defined by water tube walls: front wall 2, side walls 3 and rear wall 4, as well as a bottom 5 formed of water tubes. Combustion air is fed into the furnace from several different levels as primary, secondary and tertiary air. There may be also other air levels. Waste liquid, such as black liquor, is led via nozzles 6 located between the secondary and tertiary air zones. During combustion, the waste liquid forms a smelt bed on the bottom 5 of the furnace, wherefrom the smelt is discharged via a smelt spout 7 adapted in the lower part of the furnace.
Above the furnace, heat recovery surfaces, i.e. superheaters 8 are provided, and the heat exchangers, a boiler bank 9 and economizers 10, follow the superheaters located above the furnace and are positioned on the side of the rear wall 4. The heat generated in the furnace is recovered in said boiler bank and economizers. On the boiler banks 9, water in saturated temperature is boiled partly into steam and in feed water preheaters 10 the water is heated by means of flue gas prior to leading the water into the steam-generating part 9 and the superheating parts 8 of the boiler. In the superheaters, the saturated steam is heated to generate steam at a higher temperature. The so-called bullnose is marked with reference numeral 14.
The water/steam circulation of the boiler is arranged via natural circulation, whereby the water/steam mixture formed in the water tubes of the walls and bottom of the furnace rises upwards via collection tubes into a steam drum 11 that is located cross-wise in relation to the boiler, i.e. parallel to the front wall 2. Hot water flows from the steam drum via downcomers 12 into a manifold 13 on the bottom, wherefrom the water is distributed into the bottom water tubes and further into the water tube walls.
A waste liquor recovery boiler is conventionally formed of the following main parts, which are illustrated schematically in FIG. 1:                A lower part 16 of the furnace, where combustion of waste liquor mainly takes place.        A middle part 17 of the furnace, where final combustion of gaseous combustible substances mainly takes place.        An upper part 18 of the furnace        A superheater zone 8, wherein the saturated steam exiting the steam drum 11 is transformed into (superheated) steam having a higher temperature. In the superheater zone or upstream thereof there is often provided a so-called screen tube system 15 that usually boils water.        A boiler bank 9, i.e. water vaporizer, wherein water at a saturated temperature is partly boiled into steam.        Feed water preheaters, i.e. so-called economizers 10, wherein the feed water flowing in the heat transfer elements is preheated by means of flue gases prior to leading the water into the drum 11 and the steam-generating parts 9 and superheating parts 8 of the boiler.        A drum (or steam drum) 11 with water in the lower part and saturated steam in the upper part. Some boilers have two drums: a steam drum (upper drum) and a water drum (lower drum), wherebetween a heat transfer element, so-called boiler tubes for boiling the water are provided.        
A bullnose, where the boiler narrows and which is a common boundary area between the furnace and the heat recovery surfaces, is located at the upper part of the furnace on the rear wall of the boiler. The bullnose is formed of a recess in the rear wall of the boiler, which recess is directed towards the front wall of the boiler. Thus, the bullnose comprises a lower wall part that is typically directed diagonally from the rear wall towards the front wall of the boiler, an upper wall part that is directed from the front wall of the boiler diagonally towards the rear wall, and a bullnose arch or tip that combines these. The purpose of the bullnose area is to protect the superheater part against direct heat radiation from the furnace and to assist the upwards flowing flue gas in turning around the corner towards the flue gas discharge conduit of the boiler so that the gases flow evenly by the heat recovery surfaces. The so-called depth of the bullnose, which plays an important part in guiding the flue gas flow into the upper part of the furnace, is e.g. in single drum boilers typically 40-50% of the total depth of the furnace, which means the horizontal length of the side wall of the furnace.
Many recovery boilers are additionally provided with screen tubes upstream of the superheaters in the gas flow direction typically horizontally at the deepest part of the bullnose. Typically, a saturated mixture of water and steam flows in the screen tubes, which is connected to the water circulation of the boiler. The purpose of the screen is to cool the flue gases to some extent before they enter the superheater zone, to prevent heat radiation from the furnace to the superheater tubes and to retain a part of so-called carry-over particles escaping from the furnace.
An abundant amount of flue gases containing various impurities, such as nitrogen oxides, are generated in the combustion of various fuels, such as black liquor. During combustion, nitrogen oxide is generated from a part of nitrogen entrained in air and fuel, while the rest of the nitrogen exits as molecular nitrogen (N2) and as small amounts of hazardous compounds such as dinitrogen oxide (N2O), ammonia (NH3) and hydrogen cyanide (HCN). Nitrogen oxides are formed via several various routes, depending on the conditions and fuels.
The purpose of methods for removing nitrogen oxides is to minimize polluting nitrogen oxide emissions and thus to maximize the portion of harmless molecular nitrogen N2, simultaneously keeping the emissions of all other hazardous compounds at a low level. Typical nitrogen oxide removal methods include fuel staging, air staging and selective non-catalytic reduction, SNCR.
Selective non-catalytic reduction is reduction of nitrogen oxide generated in combustion by addition of a reagent, such as ammonia. The efficiency of the method is influenced by operation conditions, the composition of the fuel and the reagent present. Thus, this technique has provided known embodiments, comprising a fuel-lean process using ammonia, [U.S. Pat. No. 3,900,554], a fuel-rich process using ammonia [U.S. Pat. No. 4,325,924], and a fuel-rich process using urea [U.S. Pat. No. 4,335,084].
SNCR variations comprise addition of a reducing agent via various flows, e.g. with reburning fuel, with air or alone. The operation of each variation is limited to precisely determined conditions. In the absence of carbon monoxide (CO), fuel-lean SNCR operates in ranges 1100-1400 K (827-1127° C.), while fuel-rich SNCR operates at higher temperatures. However, carbon monoxide is present in almost all processes utilizing the SNCR-method, and the detrimental result thereof is shifting and narrowing of temperature windows. Optimal conditions for SNCR are hard to create in several combustion apparatuses.
U.S. Pat. No. 5,820,838 describes a circulating fluidized bed boiler, where heat transfer pipes, such as omega-pipes, are installed in the flue gas flow. In the solution, means for injecting an agent that reacts with nitrogen oxides (e.g. ammonia or urea) are integrated in the omega-pipes. The aim is to obtain adequate cooling of the reducing agent to a low temperature, e.g. 100-600° C., while injecting so that the reducing agent does not decompose. However, in this patent no attention has been paid to creating a suitable temperature window between nitrogen oxide and the reducing agent.
Decreasing of NOx-contents in recovery boilers has already been applied by methods based on staging or SNCR-technique using i) “quaternary air” in the upper part of the recovery boiler at a high level, in one embodiment of which ammonia is added entrained in said air (WO 97/21869), ii) “vertical air staging” [FI 101420 B], where air jets are fed into the furnace of the recovery boiler by means of nozzles located at several vertical elevations, iii) “Mitsubishi Advanced Combustion Technology” (MACT) [Arakawa Y., Ichinose T., Okamoto A., Baba Y, Sakai T., in Proc. of the Int. Chemical Recovery Conf., Whistler, British Columbia, Jun. 11-14, 257-260, 2001], where a reducing agent (urea) can be added after air staging, and iv) black liquor staging [FI Patent 103905], where black liquor is fed from at least two levels into a furnace having vertical air staging according to (ii). By means of these techniques, a NOx-reduction of 30-50% has been reached, but in practice they require adjustments that are not optimal for a recovery boiler. Often these techniques require oversized furnaces for keeping the temperature after the furnace adequately low and/or more expensive material solutions for preventing corrosion. In practice staged combustion or SNCR-technique in recovery boilers requires temperatures even as low as 850-1000° C., which are reached only in such recovery boilers that are bigger and thus more expensive than conventional boilers.